Coordinated multi-point (CoMP) is a feature introduced for Long Term Evolution (LTE) Advanced. CoMP enables a dynamic coordination of downlink (DL) connections and uplink (UL) connections with several different network points, e.g. antenna points, for a wireless device.
Coordination can take many forms and DL coordination schemes are typically classified into different DL CoMP modes. In 3GPP, several DL CoMP modes are being discussed and will be supported in the specification as of LTE Release 11. In the following, three examples of CoMP modes, which are among the most commonly discussed in the context of LTE Release 11, will be briefly described. Within a CoMP enabled communication network the transmissions from transmission points (TPs) are coordinated.
FIG. 1a illustrates a first example of a CoMP mode, comprising Dynamic Point Selection (DPS). DPS may be implemented in a communication network 1 comprising several TPs 2, 3. A TP 3 is dynamically selected to serve a wireless device 4 within a group of cooperative candidate TPs 2, 3. The selection may be based on different criteria, and is typically aimed at improving the DL throughput for the wireless device 4 or the overall system performance in general. The selected TP for the wireless device 4 may change dynamically, based e.g. on which TP has the highest received signal to interference plus noise ratio (SINR).
FIG. 1b illustrates a second example of a CoMP mode, comprising Dynamic Point Blanking (DPB). In DPB, the network 1 may actively and dynamically mute data transmissions from one or more TPs 2, 3 in order to reduce the interference experienced by wireless devices 4, 5 that are scheduled for DL transmission in neighboring cells.
FIG. 1c illustrates a third example of a CoMP mode, comprising Non-coherent Joint Transmission (NJT). In this CoMP mode more than one TP 2, 3 transmit the same data signal simultaneously to the wireless device 4. The wireless device 4 receives a superposition of the desired signals that has propagated over the different TP specific channels. The jointly transmitted signal can raise an average ratio between signal and noise plus interference. As a consequence, the DL signal quality is improved.
In LTE, a wireless device is expected to measure the DL radio link quality and send reports with this channel state information (CSI) to network nodes, e.g. eNodeB. For each link between a TP and the wireless device, CSI includes Channel Quality Indicator (CQI) (wideband or sub-band CQI), Rank Indicator (RI) and Precoding Matrix Indicator (PMI). In LTE the CSI content and its measurements depends on what DL transmission mode (TM) the wireless device is configured with. For instance, in TM 1 or 2, there is only a CQI measurement and report. In TM3, there are CQI and RI measurement reports. In TM4 and TM9, the wireless device should measure and report CQI, RI and PMI on a serving DL link. TM 10, introduced in LTE Rel.11, supports multiple so called CSI Processes. Each CSI Process specifies an interference measurement and a channel measurement that is used for a corresponding sequence of CSI reports (CQI, RI, and PMI). The CSI Processes may assume different serving TPs, implicitly given by the CSI-RS, as well as different interference hypotheses, given by the CSI-IM (IM, Interference Measurement) measurement. For transmission modes in LTE, refer to 3GPP Specification 36.814.
In Rel.11, the CoMP modes described with reference to FIGS. 1a, 1b and 1c are to be introduced. CSI in any of the mentioned CoMP modes have to include measurements for more links from the cooperating TPs to enable the effective operations of these CoMP modes. For instance, for DPS, accurate CSI is highly valuable and include link information from all cooperating candidate TPs so that the communication network can select the best TP to serve the wireless device. For NJT, some form of CSI regarding the estimated composite channel from at least two cooperating TPs serving the wireless device is instrumental in order to perform accurate link adaptation.
For DPB, CSI for the cases when blanking is enabled as well as disabled it is important for the network node, in particular the eNodeB, to determine an accurate modulation and coding scheme (MCS) and to capture the blanking gain.
In LTE, the reported CQI represents a recommended transport format that, if scheduled for DL transmission, should result in a specific average block error rate (BLER). These CQI reports are however prone to systematic errors, partly due to properties of the radio channel, which the eNodeB should take into account when performing link adaptation. The eNodeB may further use other sources of CSI to improve the link adaptation, such as reference signal received power (RSRP) measurements and/or channel reciprocity measurements, all with a varying degree of imperfections.
In order to ensure that a target BLER is fulfilled regardless of the CSI imperfections, the eNodeB commonly employs an outer loop link adaptation (OLLA) algorithm.
Currently, a so-called jumping algorithm is widely used for OLLA in the wireless communication systems. Based on Hybrid Automatic-Repeat-Request Acknowledgement/Negative acknowledgement (HARQ ACK/NACK) feedbacks from the wireless device, the eNodeB adjusts a radio quality compensation value (referred to as delta value or SINR adjustment value hereinafter), which is used to correct the DL radio channel quality measurement report from the wireless device or the estimated DL radio channel quality by the eNodeB. Below is a formula that exemplifies the jump algorithm:
                              Δ          delta                =                  {                                                                                          Δ                    delta                                    -                  StepSize                                                                              (                                      HARQ                    ⁢                                                                                  ⁢                    NACK                    ⁢                                                                                  ⁢                    is                    ⁢                                                                                  ⁢                    received                                    )                                                                                                                          Δ                    delta                                    +                                      StepSize                    ·                                                                  BLER                        tgt                                                                    1                        -                                                  BLER                          tgt                                                                                                                                                                  (                                      HARQ                    ⁢                                                                                  ⁢                    ACK                    ⁢                                                                                  ⁢                    is                    ⁢                                                                                  ⁢                    received                                    )                                                                                        Equation        ⁢                                  ⁢        1            
Where Δdelta is the delta value for SINR adjustment, typically in dB scale; BLERtgt is the target BLER.
The eNodeB estimates the wireless device measured DL SINR (SINRcqi) from the CQI report received from the wireless device. By using the OLLA SINR adjustment Δ, the eNodeB can derive the adjusted DL SINR (SINRadjusted) that is finally used for MCS selection and radio resource assignment. The adjusted DL SINR can be expressed as the following formula, all in the logarithmic scale:SINRadjusted=Δ+SINRcqi  Equation 2
As a consequence, the eNodeB selects the downlink MCS and assigns the corresponding radio resources taking the corrected radio channel qualities into account. If the OLLA converges properly this procedure will eventually ensure that the target BLER is reached.
In many present and future wireless communication systems, a wireless device may thus be served in parallel by the one or multiple transmission points, over one or many carriers or sub-bands, for example in accordance with the earlier described CoMP transmission modes, but also by multi-user multiple-input multiple-output (MU MIMO) techniques or carrier aggregation. In order to make full use of the downlink radio resources it is important to maintain robust and accurate link adaptation. For such a wireless device, the different radio channel characteristics of different transmission states can be either similar or quite different, which may result in, for instance, considerable differences in accuracy and/or life time of the reported CSI. Link adaptation has become increasingly more challenging due to such changing radio channel characteristics that depend on the transmission scheme. Since each specific configuration of the transmission scheme often means distinct radio link characteristics, the existing link adaptation methods are inadequate and improvements are needed to capture these circumstances.